The present invention relates to a lead and arsenic free, and preferably fluorine free optical bismuth oxide glass containing germanium oxide, to the use of such a glass in the fields of mapping, projection, telecommunication, optical communication engineering, mobile drive and laser technology, as well as to optical elements respectively preforms of such optical elements. The glass according to the present invention can also be used in the field of micro lens arrays for e.g. CCDs (charge coupled devices, such as e.g. semiconductor elements for image transformation).
In the recent years, the tendency on the market in the field of optical as well as opto-electronic technologies (application fields mapping, projection, telecommunication, optical communication engineering, mobile drive, laser technology and micro lens array) goes more and more into the direction of miniaturization. This can be seen with the finished products which become smaller and smaller and naturally requires an increasing miniaturization of the single structural members and components of such finished products. For the producers of optical glasses this development means a clear decrease of the demanded volumes of rough glass in spite of increasing quantities of finished products. At the same time, there is an increasing pricing pressure from the side of the reprocessors to the producers of glass, since with the production of such smaller components made of block and/or ingot glass noticeably more waste will be produced proportionally based on the product and for the processing of such miniature parts a higher operating expense is necessary than for larger structural members.
Instead of removing of glass portions for optical components from block or ingot glass, which is common till today, recently production procedures became important in which directly after the glass melt preforms may be yielded which preforms are as close as possible to the final contour respectively geometry, such as e.g. gobs or spheres. For example, the reprocessors' requests for preforms which are close to the final geometry for re-pressing, so-called “precision gobs”, are increasing. Normally, these “precision gobs” preferably mean completely fire-polished, free or half-free formed glass portions which are already portioned and have a geometry which is close to the final form of the optical component.
Such “precision preforms” may preferably also be converted into optical elements such as lenses, aspheres, micro lens arrays etc. by the so-called “precise pressing” or “precise molding” or “precise blank pressing”. Then, a further processing of the geometric form or the surface with e.g. a surface polish is no longer required. This procedure can comply with the smaller volumes of melted glass (distributed on a high number of small parts of material) in a flexible way by shorter set-up times. However, because of the relatively lower number of parts per time unit and the normally smaller geometries, the creation of value cannot be caused by the value of the material alone. Rather, the products have to leave the press in a state ready for installation, i.e. laborious post-processing, cooling and/or cold reprocessing must not be necessary. Because of the required high accuracy of geometries, precision instruments with high grade and therefore expensive mold materials have to be used for such a pressing procedure. The lifetimes of such molds massively affect the profitability of the products and/or materials produced. A very important factor for a long lifetime of the molds is a working temperature which is as low as possible, but which can only be lowered to a point at which the viscosity of the materials to be pressed is yet sufficient for the pressing procedure. This means, that there is a direct relationship between the processing temperature and therewith the transformation temperature Tg of a glass to be processed and the profitability of such a pressing process: The lower the transformation temperature of the glass, the longer the lifetimes of the molds; and therefore the higher the earnings. Thus, there is a demand for so-called “low Tg glasses”, i.e. glasses having low melting points and transformation temperatures, i.e. glasses with a viscosity at temperatures which are as low as possible which is sufficient for processing.
Further, from a process technical point of view of the melt there is a growing demand for “short” glasses, i.e. glasses having a viscosity which varies strongly within a certain viscosity range at a relatively small change in temperature. This behaviour has the advantage in the melting process that the times of hot forming, i.e. the closure times of the molds, can be decreased. Because of that, on the one hand the throughput will be increased, i.e. the cycle times will be reduced. On the other hand, because of that also the mold material will be protected which also has a positive effect on the total production costs, as described above. Such “short” glasses have the further advantage that also glasses with higher tendency to crystallization may be processed by the faster cooling than with corresponding longer glasses. Therewith prenucleation which could cause problems in succeeding steps of secondary hot forming will be avoided. This presents the possibility that such glasses may also be stretched to fibres.
Furthermore it is also desirable that, besides the mentioned and the required optical properties, the glasses are sufficiently chemically resistant and have an expansion coefficient which is as low as possible.
The prior art already describes glasses with similar optical state or with a comparable chemical composition, but these glasses have immense disadvantages. In particular, many of the glasses contain higher proportions of SiO2 which is a network forming agent and therefore increases the transformation temperature of the glass, creates a longer viscosity curve and reduces the refractive index, and/or components such as e.g. F and P2O5 which readily can evaporate during the melting and burning process, thus an exact adjustment of the glass composition is difficult. This evaporation is also disadvantageous during the pressing method, wherein the glass is heated again and may deposit at the surface of the molds and on the glass.
JP 2002/201039 describes a Bi2O3 containing glass with high refractive index for press molding. However, the basic glass type only contains small amounts of GeO2.
JP 04-106806 comprises a dielectric composite. The glass ingredient contains in every case CeO.
The documents WO 99/51537, JP 2001/213635, WO 01/55041, WO 03/022764, DE 10 144 475 and WO 03/022755 describe optically active glasses which in every case contain optically active rare earths.
WO 03/022763 and WO 03/022766 describe optically active glasses which are doped with at least one optically active rare-earth element and which also may contain bismuth oxide and germanium oxide, wherein however the ratio of these oxides is at least 10 for the exactly described glasses which actually contain germanium oxide as a component, i.e. the glasses have a relatively high content of bismuth oxide. According to WO 03/022766, all glasses are melted in a platinum crucible which in every case will result in the fact that the glasses contain a platinum component in amounts of higher than 3 ppm which has a negative effect on the position of the UV edge of the glasses.
DE 10 308 476 describes a bismuth containing glass which in every case contains the components B2O3 respectively SiO2, the sum of which is at most 5% by mole. SU 876572 describes an optical glass for acoustic-optical devices. However, it contains in every case more than 22% by weight of GeO2.